Just as a informal salutation to some friend. With a cheerful and playing tone in the voice. It shows friendship mostly. Also "αγόρι μου" is used to underestimate somebody of the same age or younger, when shouting each other, in an ironic tone of voice.
Generally speaking, it is like "boy" in English. Not used by elders though. A modern phrase. I think it is much influenced from English.
Αγόρι comes from the adjective άωρον (παιδίον) =immature (child) in Ancient Greek.

I've never in my life said the phrase «αγόρι μου» to my brother or any of my very close friends. I don't think I've used the word «αγόρι» as a salute either.

Some girls/women can say that to a stranger, or you can say it, without the «μου» to a waiter, but it isn't that polite.

Between close friends the most common salute is «έλα ρε μαλάκα/μινάρα/γρόθε, τι λέει;». Μαλάκα is international, μινάρα is used in Patras, γρόθε is used in Crete. I think in Northern Greece they use μαλάκα with a strong lambda (think of Russian) or καρντάσι.

If you want to underestimate or insult a man you can say «αγοράκι» or «αγοράκι μου». Because «αγοράκι» means small boy. So not only you turn a man to a boy, you turn the boy to a small boy.

Yes, I think this slung words are used very frequently among the youngsters. I would say extremely frequently, almost with the meaning of "pal". It is very rare to be used with an intimate tone in voice among elders, as the latter take these word as much insulting, with the literal meaning of these words, a masturbator and concern the sexuality of the person. To say that my father is never using these words, and mother of course. But today, it is very common to be used almost in every sentence between friends. If you use them to a foreigner it is an insult. This is an example that a word is changing meaning according the audience to which you say it. Of course they are never used in a lecture :)

Hi gusbemacbe. Yes, using the plural form «τι κάνετε» is considered more formal than the singular form. And is usually preferred when talking to strangers, or when there's an age difference (and you're the younger). At the university for example it's often used when you talk to your professor. Some people don't do it, but I think it's better to play it safe until you get to know the other person or till (s)he suggests you could use the singular (if you wanted).

A lot of operating systems can let you set up different keyboard layout one for Greek one for Spanish one for French etc. How you go about doing it is different on each operating system google search how to attach different keyboard layout to your computer and then you switch between keyboard layout when you’re doing English or doing a different language.

By the way, do lingo automatically picks the correct keyboard layout if you have it installed on your system. So if the lingo wants you to type in English it shows you the English keyboard. If the lingo wants you to type in Greek it’ll switch to the Greek keyboard if you have it on your system. So you don’t have to switch keyboards in duolingo it does it for you.